1. Field of the Invention
The invention relates to a female terminal fitting, such as an electrical connection fitting used in a connector of an automotive wiring harness and that can be connected electrically to a male electrical connection fitting.
2. Description of the Related Art
Electrical wiring, such as an automotive wiring harness, conventionally has a compression connection terminal composed of a male terminal with a plate-like tab and a female terminal fitting into which the male terminal tab is inserted. The female terminal fitting has a spring piece that compresses and holds the tab of the male terminal. The male terminal tab and the female terminal fitting are formed of copper alloy or the like. Metal plating often is applied to surfaces to avoid a reduction in connection reliability due to oxidation of contact portions. Gold plating and hard gold plating are excellent in that electrical resistance is low and an increase of an electrical resistance value caused by surface oxide formation is hardly seen even under varying temperature and humidity conditions.
Microscopic uneven structures normally are present on metal surfaces, and the area of true contact where metals are actually in contact is very small as compared with an apparent contact surface at a contact portion of the male terminal fitting and the female terminal fitting. The location of a true contact point on the apparent contact surface is difficult to specify and control when flat metal plates are in contact with each other. As a result, safety and reliability of electrical connection are difficult to guarantee. Some small female terminal fittings sandwich a tab of a male terminal between a resilient contact piece and a contact surface of a non-resilient flat plate that faces the resilient contact piece. However, the position of a true contact point of a flat inner facing contact surface is particularly difficult to specify and control.
A conventional female terminal fitting is shown in FIG. 8. A resilient contact piece 82 includes a dome-shaped embossed contact portion 82a that achieves point contact with a male terminal tab 89 shown by imaginary line in FIGS. 8(A) and 8(B), but an inner facing contact surface 81 is flat. Macroscopically, the inner facing contact surface 81 is entirely in contact with the upper surface of the male terminal tab 89. However, a force of the embossed contact portion 82a of the resilient contact piece 82 for pressing the male terminal tab 89, i.e. a contact load, should be comparable to a contact load given by the male terminal tab 89 pressing the inner facing contact surface 81. The contact load is a product of a true contact area and a contact stress, and the contact stress is determined by the hardness of a material metal and, when plating is applied, by the hardness of a plating metal.
If the resilient contact piece 82 and the inner facing contact surface 81 are formed of the same base material metal and plating metal, a true contact area on the inner facing contact surface 81 should only be about the same as the area of a top part of the embossed contact portion 82a on the resilient contact piece 82 regardless of how large an apparent contact area is. Since the true contact point is formed anywhere on the microscopic contact surface on the inner facing contact surface 81 with uniform possibility, plating conventionally has been applied to the entire inner facing contact surface 81. Applying plating selectively to only the embossed contact portion 82a on the resilient contact piece 82 is technically difficult. Thus, plating has been applied to the entire resilient contact piece 82.
U.S. Pat. No. 6,547,608 attempts to solve a problem of being unable to control the position of the true contact point on the flat surface by providing convex contact points on a resilient contact piece and a facing female terminal plate portion and ensuring a point contact or a line contact. U.S. Pat. No. 7,195,495 provides a convex portion on a surface of a female terminal fitting facing a male terminal fitting to apply a concentrated load when inserting and withdrawing the male terminal fitting so that foreign matter sandwiched between the male and female terminal fittings is removed. Japanese Unexamined Patent Publication No. 2006-172877 discloses a step projecting from a receiving portion of a female terminal fitting to be brought into contact with a male tab to solve a problem that plating comes off due to sliding abrasion between male and female terminal fittings. The step is not exposed even if plating is peeled off by applying plating to form a layer thicker than a projection distance of the step.
As disclosed in these references, plating normally has been applied to the entire inner surface even if a convex structure is formed on the inner surface of the female terminal fitting to specify the contact point between the female and male terminal fittings.
As described above, gold plating for improving contact reliability need only be applied in the vicinity of the very small true contact point. Plating applied to other parts increases material cost and facility cost for plating, but does not contribute to contact point formation. The added cost for excess plating is a particular drawback when plating with a precious metal material, such as gold.